1. Field of the Invention
This invention relates generally to voltage boost circuits, and more particularly to a metal-oxide-semiconductor (MOS) voltage boost circuit which is simple, occupies little silicon area and which substantially eliminates leakage current problems.
2. Description of the Prior Art
The advantages offered by NMOS technology are well known; e.g. higher density, greater yield, etc. Thus, smaller NMOS device geometries permit a greater number of devices to be produced per unit area or, stated another way, a single NMOS device will occupy less space. This characteristic is extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "on/off" or "one/zero" nature, most measurements in the real world are inherently analog; e.g. temperature, pressure, speed, voltage, etc. Therefore, it is necessary that microprocessors and other digital circuitry communicate or interface with analog circuitry such as amplifiers, buffers, comparators, etc., in order to permit digital processing of analog signals. The required interfacing may be accomplished by providing analog components which are external to the microprocessor chip. However, such arrangements are generally more expensive, require more current, a larger power supply and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, complex analog circuits such as voltage boosters are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing process be employed. To avoid undesirable attenuation of analog signals due to threshold drops, voltage boost circuits are often necessary. A voltage boost circuit included on an NMOS microprocessor chip must be fabricated in accordance with NMOS processing techniques, and the design of the voltage boost circuit must be tailored to such processing techniques.